> how one can filter background noises from a certain wav file
> using ecasound/ecawave?

It's really hard to give detailed guide lines for this. If the
noise is limited to narrow spectrum area, you can use ecasound's
filters (bandpass, bandreject, etc) to filter out that range.
If the noise is spread around the spectrum, you're better of
trying Matti Koskinen's denoi, dnr and anoi utils (see
http://www.linuxsound.at ).

> Another simple question: How can I fade in/fade out such
> that a sound track does not start and stop abruptly?

Ecawave 0.2.2r2 now supports fade-in and fade-out. If you want
to do it with ecasound: